CVE-2024-26938 drm/i915/bios: Tolerate devdata==NULL in intel_bios_encoder_supports_dp_dual_mode()

In the Linux kernel, the following vulnerability has been resolved:

drm/i915/bios: Tolerate devdata==NULL in intel_bios_encoder_supports_dp_dual_mode()

If we have no VBT, or the VBT didn’t declare the encoder
in question, we won’t have the ‘devdata’ for the encoder.
Instead of oopsing just bail early.

We won’t be able to tell whether the port is DP++ or not,
but so be it.

(cherry picked from commit 26410896206342c8a80d2b027923e9ee7d33b733)